In satellite communication systems and other communication systems operating at microwave frequencies, it is known to use single and dual mode parabolic reflector antennas and single mode array antennas. In many applications, it is typical to employ communication systems which have a multitude of channels in a given microwave frequency band, with each channel being at a slightly different frequency than adjacent channels. Typically, the implementation for such multiple channels involves the use of a contiguous multiplexer driving a single mode array antenna.
To minimize interference between microwave signals in or near the same frequency range, it is known to polarize the electromagnetic radiation, for example to have horizontal polarization for one signal and to have vertical polarization for another signal. In such systems, the two types or modes of polarized signals are achieved by providing two separate antenna systems, often side by side, which may use a common reflector, but have two separate, single mode, radiating arrays. Often the two antenna systems are designed to have identical coverage in terms of the far-field pattern of the beams produced by the antenna systems.
In contrast, the present invention is directed toward providing technique for minimizing interference between a plurality of independent microwave signals having the same polarization, which are being simultaneously transmitted to the same geographic location in the same general frequency band when each of the signals have the same polarization. Also, the antenna system of the present invention does not require the use of any reflectors, but instead typically uses a direct-radiating phased array antenna.
Much is known about array antennas, and they are the subject of increasingly intense interest. Phased array antennas are now recognized as the preferred antenna for a number of applications, particularly those requiring multifunction capability. Array antennas feature high power, broad bandwidth, and the ability to withstand adverse environmental conditions. A number of references have analyzed the mathematical underpinnings of the operation of phased arrays. See, for example, L. Stark, "Microwave Theory of Phased-Array Antennas--A Review", Proceedings of the IEEE, Vol. 62, No. 12, pp. 1661-1701 (December 1974), and the references cited therein.
Various combinations of radiating elements, phase shifters and feed systems have been employed to construct phased arrays. The types of radiating elements used have included horns, dipoles, helices, spiral antennas, polyrods, parabolic dishes and other types of antenna structures. The types of phase shifting devices have included ferrite phase shifters, p-i-n semiconductor diode devices, and others. Feed systems have included space feeds which use free space propagation and constrained feeds which use transmission line techniques for routing signals from the elements of the array to the central feed point. The constrained feeds typically employ power dividers connected by transmission lines. The number and type of power dividers used depends upon the precise purpose to be served with consideration given to power level and attenuation. Types of constrained feeds include the dual series feed, the hybrid junction corporate feed, parallel-feed beam-forming matrices such as the Butler matrix, and others. Large arrays at times have used a feed system which includes a Butler matrix feeding subarrays of phase shifters. As far as the inventors are presently aware, all of these features have been developed for single mode phased arrays.
The development of the Butler matrix around the very early 1960's prompted a number of generalized investigations of conditions for antenna beam orthogonality and the consequences of beam correlation at the beam input terminals. In J. Allen, "A Theoretical Limitation on the Formation of Lossless Multiple Beams in Linear Arrays", IRE Transactions on Antennas and Propagation, Vol. AP-9, pp. 350-352 (July 1961), it is reported that in order for a passive, reciprocal beam-forming matrix driving an array of equispaced radiators to form simultaneous, individual beams in a lossless manner, the shapes of the individual beams must be such that the space factors are orthogonal over the interval of a period of the space-factor pattern. The term "space-factor" refers here to the complex far-field of an array of isotropic radiators. In particular, Allen shows that array excitations associated with one input port must be orthogonal to the array excitations for any other input port. If two network inputs are identified as a and b, and if the corresponding excitations at the ith element of the array are a.sub.i and b.sub.i respectively, then the excitations are orthogonal when ##EQU1## where b.sub.i * is the complex conjugate of b.sub.i.
Allen goes on to show that each input port corresponds to an individual beam and that since the array excitations of one port are orthogonal to those of all other ports, then the individual beam associated with a port is orthogonal to all other individual beams associated with other ports. In S. Stein, "On Cross Coupling in Multiple-Beam Antennas", IRE Transactions On Antennas and Propagation, Vol. AP-10, pp. 548-557 (September 1962), there is presented a detailed analysis of the cross coupling of between individual radiating elements of an array as a function of the complex cross-correlation coefficient of the corresponding far-field beam patterns. Special emphasis is given in the Stein article to lossless, reciprocal feed systems.
In each of the foregoing references, only single mode arrays are discussed. The composite beam produced by a single mode array is typically formed from a plurality of individual beams each associated with one of the radiating elements of the array, through constructive and destructive interference between the individual beams, with the interference occurring principally, if not entirely, in space. Even in array antenna systems which employ frequency division multiplexing or time division multiplexing in order have multiple communication channels, the composite beam which is produced is of the single mode variety since only one information-bearing input signal is provided to the feed network driving the antenna array. Moreover, all of the individual beam signals, and thus the composite beam as well, share a common electromagnetic polarization.
In commonly assigned U.S. Pat. No. 3,668,567 to H. A. Rosen, a dual mode rotary microwave coupler with first and second rotatably mounted circular waveguide sections, has first means for launching counter-rotating circularly polarized signals in the first waveguide section, and second means for providing first and second linearly polarized output signals at first and second output ports. The microwave coupler provides an improved and reliable coupling device for applying a pair of output signals from a spinning transmitter multiplexer system through a rotatable joint to a pair of input terminals of a de-spun antenna system such that the signals are isolated during transmission through the coupler, thereby simplifying the design of the multiplexer system. The signals applied to the two input terminals of a two horn antenna system have a phase quadrature relationship, and each includes components from both output signals. As used therein, the dual mode feature refers to the provision of two independent antenna terminals, each providing the same gain pattern and polarization sense, but having differing senses of phase progression across the pattern.
In commonly assigned U.S. Pat. No. 4,117,423 to H. A. Rosen, a similar, but more sophisticated dual mode multiphase power divider having two input ports and N output ports, where N is typically an odd integer, is disclosed. The power divider provides a technique for providing two isolated ports to a single antenna, with the signal from each input port being called a mode and generating nearby the same beam pattern of the same polarization, but with opposite sense of phase progression for each of the two modes. As in the previous patent, counter-rotating circularly polarized signals are launched from the input ports through a cylindrical waveguide member to the output ports. In the preferred embodiment, an N-bladed septa is disposed near the second or output end of a cylindrical waveguide member to enhance the power division and impedance matching between the N output ports.
In both of these patents, the output ports are connected to a plurality of linearly disposed offset feeds at the focal region of the reflector. Specifically, in order to provide a far-field pattern having the same coverage, output signals with equal and opposite phase progressions are placed equidistantly from and on opposite sides of the focal point of the reflector. It is only by using such an off-center feed design in conjunction with a suitable (e.g., parabolic) reflector that the transmission systems described in these two patents are able to provide two modes having substantially the same coverage. It is also worth noting that the excitation coefficients of the output signals are all of equal amplitude and differ only in phase.
To the best of our knowledge, no one has developed or suggested a direct-radiating array antenna system which can be arranged so as to permit dual mode operation. As used herein the term "dual mode" of operation refers to the simultaneous transmission (or reception) of two (or more) distinct composite far-field beams of the same polarization sense in the same general frequency band wherein the composite beams have differing electromagnetic characteristics which enable them to be readily distinguished from one another.
It is the primary object of the present invention to provide a dual mode array antenna system which can produce substantially identical far-field radiation patterns for two composite beams whose excitation coefficients are mathematically orthogonal to one another. Another object is to provide a substantially lossless, reciprocal constrained feed system for such a dual mode array antenna in the form of distribution network made up of passive power-dividing devices and phase-shifting devices interconnected by simple transmission lines. One more object is to provide such a distribution network having a single separate input (or output) port for each distinct information-bearing signal to be transmitted (or received) by the array antenna system.